Thread: Confused about non-specific line-height call 1 what? px, em, ??

Confused about non-specific line-height call 1 what? px, em, ??

Looking at the style.css file, I don't understand the non-specific use on "Line-height:1"
Is there a css default for "1"?
Is this good code?
From the style.css file:

.header_menu_res ul { z-index:99; margin:0; padding:0; list-style:none; line-height:1; }